ESTRO 37 Abstract book

S1067

ESTRO 37

respectively. Breast size categories were defined using the distance between the medial to lateral tangential field borders (BS). The same 22 atlases were sub- categorised according to breast wedge inclines: 20°, 15° and 10°, consisting of 1, 11 and 10 atlases each. Auto- outline accuracy compared to 3 “gold standard” reference outlines was measured using the Jaccard conformity index (JCI). Results The JCI comparison between breast wedge incline and breast separation sub-categories showed no significant difference. Median volume, JCI values, and centre of mass for the wedge incline and breast separation categories for the heart, brachial plexus, left breast CTV, humeral head, internal mammary chain, lung and inter- pectoral, level 1, 2, 3 and 4 lymph node CTVs are demonstrated in figure 1 (a-b) and figure 2 (a-c) respectively. JCI values for the heart, breast CTV, humeral head and lung were above 0.8, and considered clinically acceptable. JCI values for level 1-4 lymph nodes were greater than 0.5 and may be clinically acceptable with minimal editing.

of a series of scores varying on OARs and connected to linearly dependent functions. The SI-DVH is defined as the average DVH generated using the minimal dose that any voxel outside the targets must receive given 100% target coverage on a set of historical CT and countered volumes for prostate patients. Material and Methods CT images and structures of 37 prostate patients, 20 treated with TrueBeam Stx and 17 with Tomotherapy unit, were analyzed through the software PlanIQ (Sun Nuclear Corporation, USA). PlanIQ quantitatively defines regions of a DVH that are impossible, and difficult to reach, based on an ideal dose falloff from the prescription dose at the target boundary. It does not require any knowledge about the delivery technique but the minimal dose to OARs is predominantly dictated by the geometric relationship between OARs and PTV, and by the prescribed dose. After having acquired all the DVHs generated by software, we created for each OAR an average SI-DVH with feasibility equal to 0.1% (impossible) and 10% (difficult). Then we created the custom-built metric, based on 21 linear functions, to evaluate plan quality by applying a linearly dependent scoring system based on the QUANTEC constraints and on the SI-DVH; we defined PQM (Plan Quality Metric), sum of all the scores on target and OARs, and APQM (Adjusted Plan Quality Metric), the maximum ideally achievable score according to patients’anatomical variability; while PQM referred to the current state of plan, APQM described its future best possible condition. We calculated PQM and APQM for the retrospective prostate plans. Applying the new SI-DVH in the optimizer as plan objective to achieve, we replanned ten prostate cases, defining DPQM as the difference between replanned PQM (rPQM) and PQM obtained previously. Results Setting the maximum PQM to 200, achievable ideally, we obtained a median PQM and APQM equal to, respectively, 133 (range:82÷177) and 154 (range:109÷189). After replanning we obtained a median rPQM equal to 161, a median rAPQM equal to 176, and an average DPQM equal to 10% (range: 5%÷16%). In terms of dose distribution, after replanning, we observed a decrease up to 10% of the dose delivered to the OAR’s. The maximum difference before and after replanning in dose distribution was observed in intermediate dose area. Conclusion SI-DVH method is a promising method which allows for simple quantification and optimization of plans against historical experiences based on contouring, protocol preparation and dose scheduling. The personalized hard score and an ideal DVH to reach could aid planners in generating treatment plans that push the limits of OAR sparing, defined by QUANTEC, that can be considered as easily achievable. EP-1961 Classical Kaposi’s sarcoma treatment with helical tomotherapy: impact of polyurethane foam cushion M. Iacco 1 , S. Saldi 2 , C. Zucchetti 1 , A.C. Dipilato 1 , V. Lancellotta 2 , C. Aristei 2 , R. Tarducci 1 1 Santa Maria della Misericordia Hospital, Medical Physics Department, Perugia, Italy 2 Santa Maria della Misericordia Hospital, Radiation Oncology Department, Perugia, Italy Purpose or Objective Classical Kaposi’s sarcoma (KS) usually involves the skin of the lower extremities and was generally treated with large parallel and opposed photon beam. To spare muscles and bones, a treatment with Helical Tomotherapy (HT) was performed. Because of the skin involvement two polyurethane foam cushions (PFC) were

Conclusion Clinically acceptable volumes can be automatically generated for the heart, breast, humeral head and lung as assessed by JCI. Similar results between atlas sub- categories were found, suggesting that in this initial study, categorisation had little effect on the atlas-based outlining system. Qualitative slice by slice assessments are required to confirm that JCI values reflect clinical assessment. EP-1960 Implementation of a statistical ideal DVH for the evaluation and optimization of treatment plans S. Bresciani 1 , F. Cresto 1 , A. Di Dia 1 , A. Maggio 1 , A. Miranti 1 , E. Garibaldi 2 , P. Gabriele 2 , M. Stasi 1 1 Candiolo Cancer Institute - FPO- IRCCS, Medical Physics, Candiolo TO, Italy 2 Candiolo Cancer Institute - FPO- IRCCS, Radiotherapy, Candiolo TO, Italy Purpose or Objective The aim of this study is to improve the quality and reduce the variability of treatment plans by developing a statistical ideal dose volume histogram (SI-DVH). From SI- DVH, we created a custom-built metric aimed to evaluate radiotherapic plans quantitatively through the definition